From an email discussion:
> I was attempting to posit that energy efficiency may be an easier rule to widely apply
> than intelligence.
Efficiency is a good metric, but it encompasses a lot more than just intelligence. Aphids might be extremely efficient in obtaining food, but that doesn’t mean they are extremely intelligent.
In fact, intelligence is remarkably inefficient, because it devotes metabolic energy to the ability to solve all sorts of problems, of which the overwhelming majority will never arise. This is the specialist/generalist dichotomy. Specialists do best in times of no change or slow change, where they can be absolutely efficient at exploiting a specific ecological niche, and generalists do best in times of disruption and rapid change.
Unlike the long and consistently warm eons of the Jurassic and Cretaceous (and the Paleocene/Eocene), the Pleistocene was defined by massive climactic fluctuations, with repeated cyclic “ice ages” that pushed glaciers all the way into southern Illinois and caused sea level to rise and fall by over 100 meters, exposing and hiding several important bridges between major land masses.
It is likely that these were conditions that favored the spread of generally intelligent species, and most likely helped select for what eventually became humans. It may not be a coincidence that the major ice sheets first began to expand ~2.6 million years ago—which is also the earliest verified date for the use of stone tools by hominids.